Peanut (Arachis hypogaea) is a common agricultural crop and one of the primary oil crops, and can be manufactured into various processed foods such as peanut candy, peanut butter, peanut oil, etc. It is known that peanuts have nutrient components such as carbohydrates, lipids, proteins, vitamin B, and minerals. Lipids compose 50% of peanuts and are primarily unsaturated lipid acids that are effective in decreasing the amount of cholesterols in human body, and preventing arteriosclerosis and cardiovascular diseases. Thus, it is healthy to eat an appropriate amount of peanut. However, the aforementioned components and effects all are linked with the fruits of peanuts. The skins of peanuts are usually regarded as useless waste.
Inventors of the present invention surprisingly discovered that the peanut skin extract is effective in increasing the expression of PTPRD gene. It is known that PTPRD gene is a tumor suppressor gene. Its deletion, mutation or low expression are found in various human tumors, such as liver cancer and gastric cancer. Based on the discovery of inventors of the present invention, the peanut skin, which was known as waste, can be used for preventing or treating liver cancer and gastric cancer.